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The answer is supported by the concept of structural reinforcement required for concrete elements, particularly in footings. When a concrete slab-on-ground is cast monolithically with its footing, there are specified guidelines regarding the placement of reinforcement.

In this scenario, the middle third of the footing is a critical area for reinforcing concrete as it’s relatively less stressed compared to the top and bottom layers. The guidelines allow certain configurations for reinforcing bars in this region. Selecting one number 5 bar in the middle third of the footing provides sufficient tensile strength to resist any bending moments that may occur in normal loading conditions.

The other options propose arrangements that may provide excess reinforcement or configurations that are not as efficient for the load distribution intended in the middle third area. The selection of a single bar specification, as in the correct choice, addresses the balance of resource efficiency while meeting structural integrity needs. This reflects sound engineering principles regarding reinforcement detailing in concrete design.